Aging in laboratory rodents is associated with a decline in regular estrous cycling, capacity to mate and ability to carry live fetuses to term. Modifications in ovarian steroidogenesis have recently been shown in nonpregnant aged laboratory rats and may be important to pregnancy failure as well. Both progesterone and estrogen are essential for implantation and optimal pregnancy maintenance in rats. However, studies correlating the pathways of ovarian biosynthesis of these steroids, their levels in the circulation and their modulation by the uterus with pregnancy failure in laboratory rats during the "subfertile" period are lacking, but essential to understanding whether they are implicated in this dysfunction. In the proposed study, four-month-old and eleven-month old female rats will be mated with young males. At several intervals during the pre- and post-implantation periods the pathway essential to the formation and metabolism of progesterone and estradiol by the ovary will be assessed by determining in vitro conversion of (3H) pregnenolone to (3H) progesterone, (3H) 20 alpha-hydroxypregn-4-en-3-one and to (3H) testosterone and of (3H) testosterone to (3H) estradiol. The capacity for ovarian progesterone and estrogen formation will be correlated with their serum concentrations and with the onset of fetal death. Gonadotropic support of the ovary will be assessed by measuring serum prolactin and luteinizing hormone levels by radioimmunoassay. Since hormonal action is modulated by levels of hormone in the circulation and by receptor content in the target tissue the concentrations of progesterone receptor in the cytosol and nuclear fractions of the uterus, which exhibit definitive patterns during normal rat gestation, will be related to pregnancy failure. To assess placental function, delta5-3beta-hydroxysteroid dehydrogenase activity, which exhibits a distinct pattern during rat gestation, will be determined in the trophoblast by in vitro conversion of (3H) pregnenolone to (3H) progesterone. Thus, a composite of hypophyseal, ovarian and uterine function will be developed, which will determine whether pregnancy failure in aged rats is preceded by, or coincides with alterations in function at these sites.